Heat transfer from a residual limb through a prosthetic device and to the exterior environment is currently limited by poor thermal conductivity of the prosthesis's liner and socket material. The closed environment of the socket of a prosthesis also prevents heat transfer through evaporation (sweating), thereby increasing skin temperature that may cause skin blisters and irritation. Many amputees report that heat and perspiration within a prosthetic socket provide significant discomfort, and the art has moved to provide certain prosthesis cooling systems.
Maintaining a normal temperature at the skin-prosthesis interface under various activities (thermal loads) is a fundamental design requirement for a prosthesis cooling system. The thermal environment inside a prosthesis is affected by heat generation of the limb (thermal load), thermal resistance of the prosthesis and ambient temperature. Hence it remains a challenge to maintain a comfortable socket temperature in various environments while performing different activities.
The prior art provides both air cooling and liquid cooling systems for personal thermoregulation. Air cooling systems based on forced convection remove metabolic heat by fan driven airflow. Due to limited contact area and low heat capacity of air, heat transfer is ineffective and therefore the cooling capacity is insufficient to remove additional heat. Instead, liquid cooling has found a wide range of applications for personal cooling in space, deep ocean, firefighting and other hazardous environments. This type of cooling system has a high coefficient of convective heat transfer which reduces the thermal resistance between a liner and the environment. Additionally, it has an easily adjustable cooling capacity controlled by the liquid's flow rate. However, comfortably using a liquid cooling system requires small diameter tubing, which translates to high power consumption for liquid circulation. Due to the relatively bulky and power intensive equipment needed for this type of liquid cooling system, a tradeoff has to be made between functionality and comfortableness.
Recently, personal cooling systems have been developed that are lightweight, compact and power efficient. Phase change materials (PCM) are a great heat storage media that can be integrated into garments where they absorb excess metabolic heat. However, controlling the cooling rate over various thermal loads remains a difficulty for PCM systems. Thermoelectric device based cooling offers better temperature control with a compact size by varying input electrical power. While it does not have any moving parts, the efficiency of currently available thermoelectric cooling devices is only between 10-15% of Carnot cycles (COP approximately 0.4-1.5). An energy-efficient cooling device is highly desired for a prosthesis cooling that can 1) maintain a constant skin temperature under all range of the thermal loads, 2) be compact and lightweight, 3) be quiet and easy to maintain.